Single Cell Omics unifies biology and technology and has become a new frontier. For mass spectrometry (MS)-based single cell proteomics and metabolomics, proof-of-principle experiments have been performed to characterize peptides and metabolites using matrix-assisted laser desorption ionization (MALDI)-MS and electrospray ionization (ESI)-MS. However, samples were processed individually offline and coverage of proteome and metabolome was limited in these studies. Key challenges still remain. First, further improvement in detection sensitivity; Second, extremely-efficient processing of minute amount of samples, down to a single cell; Third, high-throughput analysis in a cost-effective manner so that a large number of individual cells can be analyzed to achieve statistical significance. Since ESI-MS, particularly nano-ESI-MS, is the dominant soft ionization method for analyzing peptides and proteins, a fully-integrated microfluidic front-end system interfaced with nano-ESI-MS may serve as a unified platform to address the above-mentioned challenges. Microfluidics enables efficient sample manipulation and processing down to the picoliter even femtoliter range. Furthermore, the robustness and adaptability of microfabrication processes enables production of massively-parallel functional modules on a single chip for high-throughput analysis.
In fact, one of the actively-pursued areas in MS has been to implement the high-quality interface between microchips and mass spectrometers. Emitters based on polymeric materials, glass, and silicon using out-of-plane processes, had been fabricated. However, hydrophobic polymers have inherently undesirable properties for electrospray, such as a strong affinity to proteins and peptides and incompatibility with certain organic solvents; glass substrates are difficult to fabricate for complex structures; and out-of-plane strategy is critically limited in producing monolithically-integrated devices. Efforts in the field have led to two commercial MS-chips: Agilent's HPLC-chip made of polyimide and Waters' “nanoTile” chip made of ceramic. However, these devices have been developed for routine liquid chromatography (LC)-MS/MS applications and lack high-throughput capabilities. Their wide adoption by the research community remains to be seen because of their high costs and requirements for vendor-designated mass spectrometers.
Performing high-throughput ESI-MS remains a challenge because MS itself has a high capital and operational cost, limiting its scalability. Furthermore, MS is a serial detection system typically capable of analyzing one sample at a time. Hence, there is a tremendous demand in developing high-throughput MS front-end systems. One approach is to implement multiple LC systems in parallel that are coupled to a single MS detector. This reduces MS down time during sample injection and loading, and hence improves MS usage efficiency. Although in its infancy, the multiple-sprayer platform has been recognized as a potential high-quality interface for high-sensitivity and high-throughput ESI-MS. “Simultaneous multiple electrosprays” had been achieved with a bundle of fused silica capillaries and photonic fibers to improve MS sensitivity. However, the former has a size in the range of millimeters to centimeters and is not suitable for conventional mass spectrometers. Furthermore, neither of them is amenable for monolithic integration on a microchip. “Sequential multiple electrosprays” using multichannel, multitrack, out-of-plane multiple nozzles, and gated multi-inlets, had been implemented for high-throughput MS. In this approach, each sample is processed by a different front-end system (e.g., LC or CE) connected to an individual sprayer. This eliminates sample cross-contamination and allows efficient coupling between various components to reduce the dead volume/time. However, these devices also have intrinsic limitations in monolithic integration.
U.S. Patent Application Pub. No. 2010/0075428 discloses an electrospray emitter comprising: a first silica nozzle extending out from a larger silica base tube; wherein the walls of the nozzle and the base tube form a monolithic whole (hereby incorporated by reference).